On 03/10/15 16:59, Valdis.Kletnieks@xxxxxx wrote: > On Tue, 10 Mar 2015 14:03:59 +0100, Yann Droneaud said: > >>> Consider the following sequence of events: >>> >>> 0. Suppose a mutex is locked by task A and has no waiters. >>> >>> 1. Task B calls mutex_trylock(). >>> >>> 2. mutex_trylock() calls the architecture-specific >>> __mutex_fastpath_trylock(), with __mutex_trylock_slowpath() as >>> fail_fn. >>> >>> 3. According to the description of __mutex_fastpath_trylock() (for >>> example in include/asm-generic/mutex-dec.h), "if the architecture >>> has no effective trylock variant, it should call the fail_fn >>> spinlock-based trylock variant unconditionally". So >>> __mutex_fastpath_trylock() may now call __mutex_trylock_slowpath(). >>> >>> 4. Task A releases the mutex. >>> >>> 5. Task B, in __mutex_trylock_slowpath, executes: >>> >>> /* No need to trylock if the mutex is locked. */ >>> if (mutex_is_locked(lock)) >>> return 0; >>> >>> Since the mutex is no longer locked, the function continues. >>> >>> 6. Task C, which runs on a different cpu than task B, locks the mutex >>> again. >>> >>> 7. Task B, in __mutex_trylock_slowpath(), continues: >>> >>> spin_lock_mutex(&lock->wait_lock, flags); > > B will spin here until C releases the lock. > > When that spin exits, C no longer holds the lock. Re-do the analysis > from this point. > Thank you for the review. I don't think B waits for C here - C holds the mutex (lock), not the internal spinlock (lock->wait_lock). I might be wrong though. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
